Polysaccharide S-7 (abbreviated S-7) is the subject of four expired patents: (1) U.S. Pat. No. 3,960,832 issued to Kelco Jun. 1, 1976 and comprising a single composition of matter claim; (2) U.S. Pat. No. 3,915,800 issued to Kelco on Oct. 28, 1975 and comprising the growth of a naturally occurring bacterial strain Azotobacter indicus (deposited as ATCC 21423) in a submerged aerated culture in a nutrient medium and recovering the polysaccharide; (3) U.S. Pat. No. 3,894,976 issued on Jul. 15, 1975 and claiming use of S-7 in water based paints; and (4) U.S. Pat. No. 3,979,303 issued Sep. 7, 1976 and claiming use of S-7 in oil well drilling. A recent US patent (U.S. Pat. No. 5,772,912) issued Jun. 30, 1998 claims use of S-7 in anti-icing formulations, and another (U.S. Pat. No. 4,462,836) issued Jul. 31, 1984 claims use of S-7 in cement. The published literature concerning this polysaccharide is limited to a 1977 review by the Kelco inventors that is based on the information in their published patents, and two brief studies by others concerning conditions for growing the naturally occurring bacterium. The parent bacterium is referred to here as Sphingomonas strain S7 since it was reassigned to a new genus (T. J. Pollock, 1993, Journal of General Microbiology, volume 139, pages 1939-1945).
First, modified derivatives of the naturally occurring parental bacterium Sphingomonas strain S7 were constructed by genetic engineering that exhibit increased conversion of the carbon source in a nutrient culture medium into the product exopolysaccharide S-7, compared to the unmodified parent strain.
Second, a polysaccharide was prepared from one of the genetically-modified derivatives that has a carbohydrate composition which is different from the parent polysaccharide S-7, and which confers increased viscosity on the polymer while in the fermentation broth or after isolation of the polymer.
At this time we do not know the chemical structure of the derived polysaccharide, so we do not know the basis for the change in physical properties.
Polysaccharides like S7 are used to modify the viscosity of aqueous solutions. Several polymers have this capacity, such as xanthan gum, cellulose, and guar. A new polymer like that produced by Sphingomonas strain S7/pRK-S7c6, which is described below and which represents a new composition of matter, shows increased viscosity at lower concentrations.